In heated or cooled buildings, fresh air or “makeup air” is typically added continuously to the total volume of circulated air. A corresponding amount of previously heated or cooled air is then exhausted from the building space. This can result in an undesirable loss of energy, as the fresh air is brought to the same temperature and humidity as the treated air that will be exhausted to the external environment. Heat exchangers are commonly used in the exhaust and makeup airflow pathways of these systems to recover some of the lost energy. Such exchangers may be used to induce warmer makeup air during heating processes and cooler makeup air during cooling processes. Moisture exchangers may also be used to either remove or add moisture to the makeup air so that it more closely matches the treated air to be exhausted.
Materials commonly used for heat exchangers include metal foils and sheets, plastic films, paper sheets, and the like. These systems can be effective at transferring thermal energy but are not effective at transferring moisture. Materials used for moisture exchangers include desiccants or other moisture adsorbing materials. These can be effective at holding moisture, but releasing said moisture into the makeup air requires additional mechanical energy to relocate the material or thermal energy to induce the desiccant to desorb the accumulated moisture.
The various types of heat and moisture exchangers in common usage are generally located at or near the building air-handling units in the mechanical room or basement, or on the rooftop of the building. The nature of moisture exchange requires a very large surface area in contact with the gas stream and, consequently, so-called total heat exchangers are often very large in size when compared to heat-only exchangers. A single, large exchanger in a conventional location requires additional space and/or additional load-carrying capacity of the roof in the case of a rooftop unit, incurring a financial penalty in the cost of the building construction or modification.
Instead of using a single, large, central unit to precondition the stream of makeup air, multiple smaller units may be used and distributed throughout the building. For example, U.S. Pat. No. 6,178,966 to Breshears describes a heat and moisture exchange apparatus configured to be integrated in the exterior walls of a building. Such units may face numerous challenges. First, creating a pathway from the external fresh air into the interior building space may require breaching the waterproof barrier around the building. Second, smaller exchange units may have difficulty adequately separating the inlet and outlet points of the makeup air stream from the outlet and inlet points of the exhaust air stream, respectively, to ensure that recently introduced fresh air is not immediately exhausted out of the building and that recently exhausted air is not immediately recirculated back into the building.